1. Field of the Invention
This invention relates to an offset voltage correction circuit for an operational amplifier.
2. Description of the Related Art
An offset voltage in an operational amplifier tends to vary in accordance with a temperature change. Also, the offset voltage tends to vary as parameters of the components of the operational amplifier changes through time in accordance with ageing-based changes in parameters of parts. It is known that during the manufacture of an operational amplifier, the operational amplifier is adjusted by a trimming device for offset voltage correction.
Japanese published unexamined patent application 5-129848 discloses an offset voltage compensating circuit for a differential amplifier. The circuit of Japanese application 5-129848 includes a short-circuiting switch SW1 to set input terminals of a differential amplifier at the same voltage, capacitors Cl and C2 connected to a node to change the output voltage of the differential amplifier, and charge/discharge switches SW3-SW6 to connect the capacitors Cl and C2 to a charging source or a discharging source. Also, the circuit of Japanese application 5-129848 includes a comparator and a switch control circuit. The comparator operates to compare an output voltage from the differential amplifier with a prescribed reference voltage. The switch control circuit operates the short-circuiting switch SW1 when receiving a compensation starting signal. The switch control circuit operates the charge/discharge switches SW3-SW6 in response to an output signal of the comparator so that the output voltage of the differential amplifier can be coincident with the prescribed reference voltage.
In the circuit of Japanese application 5-129848, charges in the capacitors C1 and C2 form balance voltages for offset voltage compensation. The capacitors C1 and C2 tend to be naturally discharged as time goes by. Accordingly, the circuit of Japanese application 5-129848 is repetitively activated to implement offset voltage compensation while the differential amplifier is used for a long term.